1. Field
The present invention relates generally to data communication, and more particularly to novel and improved techniques for adjusting a target received signal quality, or setpoint, for a power control loop in a wireless communication system.
2. Background
In a wireless communication system, a user with a remote terminal (e.g., a cellular phone) communicates with another user via transmissions on the forward and reverse links with one or more base stations. The forward link refers to transmission from the base station to the remote terminal, and the reverse link refers to transmission from the remote terminal to the base station. The forward and reverse links are typically allocated different frequency bands.
In a Code Division Multiple Access (CDMA) system, the total transmit power from a base station is typically indicative of the total capacity of the forward link since data may be transmitted to a number of users concurrently over a shared frequency band. A portion of the total transmit power may be allocated to each active user such that the total aggregate transmit power for all users is less than or equal to the total available transmit power.
To maximize the forward link capacity, the transmit power to each remote terminal may be controlled by a first power control loop such that the signal quality, as measured by the energy-per-bit-to-total-noise-plus-interference ratio (Eb/Nt), of a transmission received at the remote terminal is maintained at a particular target Eb/Nt. This target Eb/Nt is often referred to as the power control setpoint (or simply, the setpoint). A second power control loop is typically employed to adjust the setpoint such that a desired level of performance, e.g., as measured by the frame error rate (FER), is maintained. The forward link power control mechanism thus attempts to reduce power consumption and interference while maintaining the desired link performance. This results in increased system capacity and reduced delays in serving users.
In a conventional implementation (e.g., as defined in the IS-95 standard), the setpoint is adjusted based on the status of received data frames (or packets). In one scheme, the setpoint is increased by a relatively large step (e.g., ΔU=1 dB) whenever a frame erasure is detected (i.e., the frame is received in error). Conversely, the setpoint is decreased by a smaller step (e.g., ΔD=0.01 dB) whenever a frame is properly decoded. For this scheme, the frame error rate is approximately equal to the ratio of the “down” step over the “up” step (i.e., FER=ΔD/(ΔD+ΔU)).
The setpoint adjustment scheme described above results in a sawtooth response for the setpoint. This sawtooth response may result in transmission at higher power level than necessary since the setpoint can only be decreased in small steps. Moreover, accurate adjustment of the setpoint to reflect changing link condition is hindered by the fixed and small adjustment steps.
As can be seen, techniques that can be used to effectively adjust the setpoint of a power control loop, which may reduce transmit power consumption and interference and further increase system capacity, are highly desirable.